In existing thermal ink jet printing, the printhead typically comprises one or more ink ejectors, such as disclosed in U.S. Pat. No. 4,463,359. Each ejector includes a channel communicating with an ink supply chamber, or manifold, at one end and has an opening at the opposite end, referred to as a nozzle. A thermal energy generator, usually a resistor, is located in each of the channels, a predetermined distance from the nozzles. The resistors are individually addressed with a current pulse to momentarily vaporize the ink and form a bubble which expels an ink droplet. As the bubble grows, the ink rapidly bulges from the nozzle and is momentarily contained by the surface tension of the ink as a meniscus. As the bubble begins to collapse, the ink still in the channel between the nozzle and bubble starts to move towards the collapsing bubble, causing a volumetric contraction of the ink at the nozzle and resulting in the separation of the bulging ink as a droplet. The acceleration of the ink out of the nozzle while the bubble is growing provides the momentum and velocity of the droplet in a substantially straight line direction towards a print sheet, such as a piece of paper. Because the droplet of ink is emitted only when the resistor is actuated, this type of thermal ink-jet printing is known as "drop-on-demand" printing. Other types of ink-jet printing, such as continuous-stream or acoustic, are also known.
In a single-color ink jet printing apparatus, the printhead typically comprises a linear array of ejectors, and the printhead is moved relative to the surface of the print sheet, either by moving the print sheet relative to a stationary printhead, or vice-versa, or both. In some types of apparatus, a relatively small printhead moves across a print sheet numerous times in swaths, much like a typewriter; alternatively, a printhead which consists of an array of ejectors and extends the full width of the print sheet may pass one time down the print sheet to give full-page images, in what is known as a "full-width array" (FWA) printer. In a second type of FWA printer, the printhead remains stationary and the print sheet passes the printhead. When the printhead and the print sheet are moved relative to each other, imagewise digital data is used to selectively activate the thermal energy generators in the printhead over time so that the desired image will be created on the print sheet.
With ink-jet printing, it is also possible to create multicolor images on a print sheet. This type of printing may be used for full-color images, such as to reproduce a color photograph, or can be employed for "highlight" color, in which colored additions are made to a main portion of the image or text, which is typically black. In addition to being able to print multicolor images or images having highlight color, it is also desirable to print successive batches of documents with different color inks or with different types of inks, such as fast drying or slow drying inks with one printing machine.
In each of these instances, it may be necessary to change the ink currently being printed to a different type of ink without having to change the printhead. In such cases, fast and accurate changing of the ink without mixing of inks is essential to printing accurate reproductions of color images.
In U.S. Pat. No. 4,908,638 to Albosta et al., an n-way selecting mechanism for selecting inks from a number of ink supply containers for delivery to the marking head of an ink jet printer is described. The selecting mechanism includes a rotary diverting valve which is positioned to allow the marking head to receive ink from one color supply container or another color supply container.
U.S. Pat. No. 5,126,752 to Weinberg describes an ink jet printer head flushing system. The flushing system includes a number of valves and lines in which solvent is delivered to a print head over an ink feed line by using a suction device and an ink pump.